Bioreactors are used for large-scale industrial production of phototrophic organisms, e.g. cyanobacteria or microalgae, for example Spirulina, Chlorella, Chlamydomonas or Haematococcus. These microalgae are able, with the aid of light energy, to convert CO2 and water into biomass. Photobioreactors of the first generation use sunlight as the light source. The reactors consist of large open tank units of various shapes, for example round tanks with diameters of up to 45 m and rotating stirring arms. These reactors are generally made of concrete or plastics. Closed bioreactors are also used in many different forms. Closed bioreactors can be plate-type bioreactors, tubular bioreactors, (bubble) column bioreactors or hose-type bioreactors. This type of reactor is made of transparent or translucent materials, such as glass or plastic.
To date, the culture conditions of phototrophic microorganisms, which are produced in closed reactors, cannot be kept constant for an extended period, as the phototrophic microorganisms that form in a culture phase are deposited on the reactor walls, which leads to fluctuations in the amount of light supplied to the culture medium and to variable mixing of the culture medium. Algal deposits are often caused by stress conditions during cultivation, the causes of which can be uncontrolled growth conditions (e.g. light, temperature in open-pond and in closed reactors) of the microorganisms or induction of the production of valuable substances by the phototrophic organisms (e.g. astaxanthin, beta-carotene).
WO 2007/129327 A1 relates to a photobioreactor for cultivation of biomass, which is constructed of transparent, helically-coiled tubes. Silicone is generally recommended as tube material, and there is no discussion of the fouling problem. The illumination of photoreactors with LED plastic moldings is described in WO 2008/145719. Steel, plastics and ceramics are listed as reactor materials. The illuminating element is preferably an LED silicone molding. WO 2004/108881 A2 includes a bioreactor arrangement of vessel and light source, and all possible plastics, including silicones that are not specified in greater detail, are stated as materials for the vessel. WO 2009/037683 A1 describes a bathtub-shaped bioreactor with umbrella-shaped cover made of transparent materials, which are not further specified. Gas-permeable hoses, preferably of silicone, are used for feed of carbon dioxide. GB 2118572 A describes a photobioreactor with glass tubes, which are joined with U-shaped connectors made of silicone. DE 10 2005 025 118 A1 describes a photobioreactor made of glass tubes, wherein microorganisms that have accumulated on surfaces are removed by means of ultrasound. US 2003/0073231 A1 describes a photobioreactor made of thermoplastics such as polyvinyl chloride or polyethylene. The object of US 2007/0048848 A1 is also a photobioreactor made from thermoplastics. In both cases, deposits of microorganisms on the reactor walls are removed by mechanical means, for example brushing. These are in all cases relatively expensive methods, which cannot be scaled up as desired. In DE 44 16 069 A1 it is recommended to provide light-conducting fibers, which are used for illuminating bioreactors, with a smooth surface. US 2008/0311649 A1 proposes increasing the flow rate of the medium containing the algae in tubular bioreactors, to prevent deposition of the algae. This has the disadvantage that the culture parameters with respect to flow rate can no longer be set independently.